The present invention relates to a method of manufacturing fins for heat exchangers; more precisely relates to a method of manufacturing fins for heat exchangers such as air conditioners.
Radiating fins for heat exchangers of air conditioners are made of rectangular metal plates, e.g. aluminum plate, with a plurality of collared through-holes with a prescribed collar height.
A heat exchanger's radiating section composed of said fins is assembled by, first, stacking a plurality of fins such that the collared through-holes are coaxial. Then, pipes made of heat conductive material, e.g. copper, are inserted through the through-holes to integrate the fins.
The conventional method of manufacturing fins for heat exchangers will be explained with reference to FIG. 7B.
The method shown in FIG. 7B comprises following steps:
forming a shallow inverted vessel section 106, which looks like a truncated cone or a column, in a metal plate, wherein the bottom diameter of the inverted vessel section 106 is greater than that of a collared through-hole to be formed [step shown in (i)];
drawing the inverted vessel section 106 so as to gradually reduce the diameter thereof while increasing the height until a first prescribed height is obtained [steps shown in (ii), (iii) and (iv)];
boring and burring the inverted vessel section 109 until a second prescribed height is obtained, thus forming a collar 104 [step shown in (v)]; and
forming a flange section 105 by outwardly bending the front end section of the collar 104 [step shown in (vi)].
In the method (drawing method) shown in FIG. 7B, high collars can be formed, but coaxial circular punch marks or wrinkles are apt to form near the collared through-holes, and the metal plate is apt to be twisted or otherwise warped during the drawing steps for reducing the diameter of the inverted vessel section.
Furthermore, as shown in FIG. 8, the corners of the inverted vessel section become relatively thinner during the drawing steps, so that the metal plate hardens and is apt to crack, resulting in greater difficulties in machining.
To solve the above described disadvantages, there is disclosed a manufacturing method in the U.S. Pat. No. 4,055,067 (see FIG. 7A). The method shown in FIG. 7A is described below:
The first step is boring and burring a metal plate 100 to form a through-hole 101, which is described by a projected section 102 [step shown in (a)].
Next, the projected section 102 is ironed to expand the diameter of the through-hole 101 so as to form a collar having a prescribed height [steps shown in (b) and (c)]. This ironing of the projected section 102 and the collar 103 is executed by an outer circumferential face of a punch and an inner face of a die.
In the method shown in FIG. 7A, the ironing has two steps, and the diameters of punches and dies are different in each step.
The front end of the collar 104, which has the prescribed height, is then bent to form a flange 105 [step shown in (d)].
In the method (ironing method) shown in FIG. 7A, the diameter of the through-hole is expanded during the ironing steps, so that no coaxial punch marks are formed near the collared through-holes. Furthermore, even if the metal plate is made of a hard metal, which is typically difficult to machine by ironing, collared through-holes having a prescribed height can be formed.
In the above described ironing method, however, the use of thinner metal plates decreases the degree of effective ironing potential due to formation of unacceptably thin collar walls, and it is therefore difficult to form high collared through-holes.
To solve the aforementioned problem, it is necessary to bore small through-holes and then form relatively high projected sections in said thin metal plates prior to ironing. The degree of reduction of the diameter of the through-hole is limited, however, because in plates having holes too small, cracks are apt to form in the collars during ironing.
Currently, thinner fins are required for economical operation and weight reduction of air conditioners, and it is difficult to machine these thinner metal plates using the convention ironing method.
On the other hand, in the drawing method, there is no limitation caused by the thickness of the metal plates, but machining is more difficult because the metal plates have a greater hardness in order to have sufficient mechanical strength.
Furthermore, for environmental protection, the usage of Freon ("Freon" is the trademark for fluorocarbons) has been restricted, and it's utilization in the removal of machining oil from the machined fins may be prohibited in the near future.
The use of volatile oils as machining oils has been proposed to eliminate the need for Freon cleaning, but the ironing degree decreases with use of volatile oils. Because of the lower ironing degree, and thinner yet harder metal plates, it is quite difficult to form the collared through-holes to the prescribed height.